Coastal Management Technical Notes The syllabus says you should learn about ·the geographical processes relevant to the issue ·the perceptions of different groups about the issue ·individual, group and government responses to the issue ·decision-making processes involved in the management of the issue ·management of the issue and implications for sustainability, social justice and equity 1. Geographic Processes
1.1 Oceanic Processes Waves Waves are created by wind blowing over the surface of water. Waves can travel thousands of kilometres until they reach shallow water where friction on the seabed causes them to slow down, become steeper, and eventually break upon the shore. Waves have a significant role in shaping the coastline, cutting into the softer rocks to carve out embayments, and leaving the more resistant rocks standing as headlands, cliffs and bluffs. Understanding waves and the currents they generate is important for beach safety, erosion control and the planning of developments and artificial structures both offshore and along the coast, such as seawalls, harbours, oil production platforms and buildings. Rips and Longshore Currents After a wave has broken on the shore, the water withdraws back to the sea either as an undertow (sheet flow over the sea floor) or in a rip current. Rip currents occur on many ocean beaches and can be extremely dangerous as a strong rip current can carry swimmers out to sea. The direction in which rip currents travel depend on the angle at which the waves approach the shore. The direction from which waves approach the shore affects where sand or other material is transported. When waves approach parallel to the shore, sand is moved in and out from a beach in cycles of fine and stormy weather, and there is generally no significant loss of sand from the area. Waves approaching at an angle to the shore generate currents which move sand in one direction along the shore. This process of longshore drifting can cause significant beach erosion, especially on long beaches uninterrupted by headlands, estuaries and inlets. Sand transported by longshore drifting is deposited further along the coast to form spits, which may later be washed away by a storm and deposited elsewhere. The western shore of Western Port and the north coast of Phillip Island are particularly subject to this as the south-westerly swell pushes into Western Port and moves sand along the shore. Erosion, Weathering and Deposition Erosion is the wearing away of cliffs, dunes, beaches and other landforms by the action of wind, rain or sea. Weathering is the breaking down of rock or soil by climatic conditions such as rainfall, hot sun or frost. Deposition is the accumulation of sediments, and is the process by which beaches, spits and mudflats are formed. Sand deposited by waves forms beaches, spits and barriers. Material eroded from cliffs also helps to build beaches. Mud and silt deposited by rivers builds mudflats on sheltered coasts. Waves are closely linked with weather and the processes of erosion and deposition along the coast. In stormy weather, waves plunge with great force upon sandy shores, pulling sand from the beach and withdrawing it to the sea. Waves attack cliffs, wearing away at the rock, breaking it down into sediments. After storms, backshore dunes may be cliffed and rocky outcrops appear on depleted beaches, yet after a period of calm weather the beach and dunes are often replenished with sand. In fine weather, spilling waves deliver sand to the shore creating and supplementing beaches, spits and sandy barriers. This cyclic pattern of cut (erosion) and fill (deposition) is constantly changing the shape of the coast in the short and longer term. Extreme Events Storm surges are a build up of water in the coastal zone by strong on-shore winds (winds blowing from the sea towards the land) and can cause severe erosion, especially when they coincide with high spring tides. A normal high tide is about 1.5-2m, a spring tide can be 3-4m and a storm surge 2-5m. If a storm surge combines with a spring tide. The sea level may be raised by 5-9m and have large 4-5m waves on top of that. This combination of conditions is rare, occurring perhaps about once every 20 years. It is also what causes the severe storm events which cause buildings to be washed into the sea. 1.2 Dune Processes 1.2.1 Formation Primary dunes are derived from the beach and constructed where colonising vegetation becomes established. These are usually frontal dunes parallel to the beach. Secondary dunes develop as erosional features from the primary dunes. In contrast to the primary dunes, secondary dunes are often aligned parallel to the wind which formed them, and therefore the dunes lie at high angles to the beach. The seaward side of most dunes have a relatively gentle slope of about 10 degrees and a lee slope or slip face which lies at about 32 degrees - the angle of repose for dry sand (ie. dry sand cannot be piled into a hill with a slope more than 32o). When winds are strong and persistent and vegetation is absent, secondary dune crests can move inland at 2 to 3m per year. A well developed vegetation pattern is usually present inland from the beach across any dune sequence. Colonising plants such as sand spinifex grow on the beach berm and frontal dune in raw sand. Inland from this are the pioneer shrub species such as coastal wattle; then low woodlands of tea tree and coastal banksia; to denser, taller woodlands and forests of eucalypts. Soil development also increases inland. In most dune systems the tall forest is seen as the end point in vegetation communities but other end points can be recognised where the systems are larger and older. At Cooloola in southern Queensland for example, the oldest inland dunes are covered in a low banksia scrub beyond the forest stage because soil nutrients have been leached from the older sands and this restricts plant growth. In places on Fraser Island and at Myall Lakes the most complex plant community are patches of rainforest where virtually all the soil nutrients are recycled from the litter layer. The plant species present in any dune system vary from place to place but the pattern is consistent and can be explained in terms of the ecological concepts of zonation and succession. Zonation being the communities’ response to gradual changes in environmental conditions from the open coast inland, for example high wind shear, salt spray, and soil development. Dunes further from the beach are older and the concept of succession may apply if each part of the dune system has passed through all the development stages. Elements of succession are certainly evident on any prograding (advancing) dune system but we should acknowledge that the processes causing dune advancement today are not necessarily the same as those operating thousands of years ago. 1.2.2 Dune Dynamics The Erosion and Accretion Cycle The physics of wind erosion is fairly simple. It occurs wherever a sandy soil is exposed to wind shear - the sand drifts and moves as mobile dunes until it is impeded by vegetation or enters a stream or lake. Dune fields vary in size depending on wind conditions, the extent of protective vegetation, and the supply of new sand from the beach. Sand supply at the beach depends on the tidal range, wave regime and the delivery of sand from rivers. Along most of the Australian coast river valleys were converted to deep estuaries by the Pleistocene sea level rise. Because the total sand supply is finite and these estuaries now trap river sand, coastal erosion is likely to occur where adjacent beaches are not being nourished by new sand. Each beach system behaves as a closed cell in which the sand on the beach and frontal dunes moves on and offshore over time depending on the wave regime. Longshore transport or exchange of sand around coastal headlands is relatively unimportant except in northern New South Wales and southern Queensland. Waves are the most important process affecting the form of the beach and frontal dune. Two types of waves occur around the coast and their distribution varies seasonally and geographically (Figure 2). Swell waves are generated by storms thousands of kilometres away and follow great circle routes around the world. On the east coast they mainly come from the southeast, on the west and south coast they come from the southwest. Dominant swells are important because swell waves are constructive waves which steadily move sand onto the shore where it may be blown into dunes. Storm waves may be generated by local storms and more distant tropical cyclones and can come from any offshore direction. They are steeper, higher, more closely spaced (shorter period) and more destructive than swell waves. In northern Australia tropical cyclones occur once or twice a year and although they have very high wind speeds waves seldom exceed 2.5m in height. Coasts facing the Southern Ocean suffer the impact of storm waves 5 to 6m high for up to 40% of the year. A typical erosion/accretion cycle on the east coast sees winter erosion by storm waves followed by slow rebuilding of the beaches and frontal dunes by the summer swell. The body of sand affected by this cycle lies within the sweep zone. Figure 3 illustrates some of the main features of a typical coastal dune sequence and a sediment budget. Soil Formation Soil profiles formed in coastal sand dunes increase in depth and maturity with time. At the frontal dune there is no soil development. However in the hind dunes, litter begins to accumulate, a topsoil forms and any shell fragments in the sand are dissolved and leached away changing the soil pH from alkaline to acid. Moving inland through the taller and more diverse vegetation, podzol soil profiles develop in the inert quartz sands. Here the only other mineral components are the thin films of iron and aluminium oxides coating the sand grains and colouring them yellow. Plants growing on these low nutrient sites leach organic solutions into the soil as rainwater percolates from the surface. These solutions dissolve the oxides coating the sand grains and this iron/organic complex penetrates deeper into the soil where it breaks down and is precipitated as dark coloured ‘pans’ cementing the sand. The result of this podzolisation process is a distinctive soil profile. The topsoil is dominated by accumulating organic matter where most of the plant nutrients are stored and recycled. Beneath this lies a deep and intensely bleached white A2 horizon where all oxides have been stripped from the sand. Below this a complex of black or red-brown iron and organic pans is found. As time passes the soil profile deepens. The pans dissolve and move down through the profile forming bleached horizons many metres thick. The well known coloured sands of Rainbow Beach and Tewantin in southern Queensland are examples of giant podzol profiles. 1.2.3 Dune Vegetation Figure 1: Coastal Sand Dune Vegetation Vegetation And Coastal Processes Vegetation is the key factor in dune stability and it is the vulnerability of dune vegetation that makes the dunes sensitive to impact. Examples of human activities that can damage dunes are: bush clearing; stock grazing; road building; construction; fuel storage; housing; most recreational activities; pedestrian and vehicular traffic; brush cutting and bushfires. Role and Values For the purposes of coastal dune management, it is convenient to divide dune vegetation into three groups. These groups are primary colonising species, (grasses and vines); secondary shrubs and transient species; and tertiary species (enduring trees and understorey).Dune vegetation is highly adapted to the salt laden winds of the coast, and maintains the foredunes by holding the sand already in the dune, trapping sand blown up from the beach, and aiding in the repair of damage inflicted on the dune either by natural phenomena or by human impact. The combination of dune height, dune shape and intact vegetation creates a protective system which directs salt-laden winds upwards and over the dune crest. As a result, salt sensitive vegetation communities, (including littoral rainforests), can establish in close proximity to the beach. Vegetation plays an important role in the stabilisation and formation of coastal dunes. Foreshore vegetation impacts on several of the sand transport pathways, and therefore influences the rate of shoreline recession and dune rebuilding. The strandline and beach vegetation is generally transient, being removed during storm events. Nevertheless these pioneer plants trap and hold windblown sand so that it does not damage plants on the relatively more stable foredune. The beach vegetation is mainly dominated by grasses. The foredune vegetation proper is usually composed of semi-permanent populations of herbs, shrubs and trees which stabilise the foredune sand mass. Sand trapped in the foredune acts as a reservoir of sand for the beach during periods of wave erosion, the foredune vegetation also buffers the effects of storm erosion.
Casuarina Primary Sand dune binding
Function of Dunes Fore dunes act as Barriers against the action of waves, tides and salt wind and are a source of sand for the beach during periods of erosion. Their colonisation and consolidation by plants gives them a flexible function as sand traps and self consolidation after exposure to heavy storm waves. Fore dunes that are densely vegetated protect hind dune habitats and promote more complex and diverse plant communities.
Secondary and tertiary dunes are the products of uninterrupted succession. These provide further protection and the establishment of those critical ecological conditions conducive to promoting the most stable of all stages the coastal climax forest dominated by advanced flowering plants such as eucalypts, coastal apple and tuckeroos.
Humble Beginnings - The infant dune and its journey Whether they be of the stable uniform stretch of low dune characteristics of our northern NSW coastline or the gargantuan sand cliffs of Cooloola and the huge packed parabolic dunes of Stradbroke, Moreton and Fraser Islands transgressing inland as wind blown deltas of fine white sand called blowouts they all started in the same humble way. The little Hillock now receives bird droppings rich in nitrates and phosphates, good plant food. Inevitably some of these beach "pimples" will stop the wind blown march of a strange spider like tumble weed, the seed capsule of that hardiest of all grasses called spinifex.
Spinifex on Primary Dune
Soon after, it germinates to establish a root hold and spreads web like rhizomes (creeping stems) to capture the hillock and other rolling tumble weeds in it's first act of sand stabilisation and dune formation. These are the pioneer colonisers of sand dunes. Colonisers like Spinifex improve local conditions for other binders such as pig face and the goastfoot vine to join the battle for stabilisation. Together with time, isolation and minimal interference they help create the first thin skin of coastal protection called a primary dune.
Morning glory, another dune colonising plant Coastal Wattle Heathland plants
A green fuzzy ridge of Spinifex and sand, forms a low natural wall of defence against intense invasion from the sea. Behind this wall a snug valley called a swale nurtures conditions for nutrients to accumulate and other specialised plants to further diversify. The salt and wind loving pioneers of the fore dune that helped establish this swale now gives way to new varieties of plants. Low sand holding shrubs like coastal wattle (acacias), banksias with their remarkable predilection for poor soils and accumulating phosphates and adaptations against salt wind and severe dehydration. Mean while in front of the primary dune new mini dunes continue to form. Over time compounded by the dropping of sea levels they become new primary dunes relegating the original dune to a highly vegetated sand hill called a secondary dune. As sea levels continue to drop over the last glacial period, tertiary dunes form. The original dune has become a coastal forest heathland and it's extended swale a luxurious insect and bird rich heath land. We must ask ourselves what would happen to the great spread of coastal urban development that have displaced so many sand dunes of their natural barrier systems if a reversal was to occur and sea levels started to rise? From the air a series of parallel beach ridges fringing the seaward edge of many coastal plains indicate a history of coastal advance and sea retreat. The highest sand hill in the world is Mt Tempest of Moreton Island it is 280 metres tall and is in fact a huge transgressive dune. This whole evolutionary give and take process of dune formation and its protective establishment and sustenance of developing coastal forest is called succession, the journey of the dune. Like the adding and shedding of our skin over time the dunes and sea cliffs form a dynamic living security system that surround our continent. It nurtures the essential feedback processes of homeostasis that keeps coastal terrestrial eco systems in balance enabling them to operate autonomously with relative impunity. Our dune system also
alike to the skin is a fragile sensitive organ that functions most effectively within the natural parameters set by it's own evolving tolerance to interference and change. The coastal forests, heathlands, wetlands and mangroves, sea grasses and salt marshes that this dune succession spawns promotes outgoing sediments of sand and detritus (waste organic matter) which unless contaminated by alien infusions of questionable origin and intent feed the capillaries of it's creek and river systems to fuel the oceans and ultimately refurbish the very basis for it's own continuously recycled and sustained existence. Key members of nature's green dune squad and their significance Higher up on the dunes it is salt winds that is the invasive force. Aerial parts of vegetation block the wind energy and cause sand to deposit around the vegetation. A characteristic of dune vegetation particularly grasses like spinnifex, pineapple sedge and dune carex is their ability to produce up right stems and sand trapping rhizomes that can grow firm roots in response to sand coverage. This sand deposition around plants results in increase height and width of the dune, a process known as plant induced dune expansion. The ability of pioneer plants such as Spinifex and Pigface to hold wind blown sand on the frontal dunes helps create conditions which encourage the establishment of other communities such as Woodland Scrub (Acacias, Tuckeroos) Heathland (Banksias, Xerophytes such as Pea plants, Heath and Boronias and Melaleucas and Coastal Forest (Eucalypts and Angophora). All plants (Herbs, shrubs ,reeds, grasses, trees) are of equal importance in developing vegetation. Like the interplay between diverse cell types within the connective tissue meshwork of our skin underlay (dermis) that stops the outer skin from splitting and falling apart these plants enable dune stabilisation to continue indefinitely. The whole process of dune formation and succession depends primarily on the pioneer plants and of all of these Spinifex is the chief. Spinifex hirsutus (The Captain Cook of the dunes). This is the most successful sand trapping dune coloniser and pioneer along our northern beaches. It provides the basis for a dune and establishes the first of a new set of environmental conditions conducive to the radiation of a more diverse vegetative cover. These conditions include increased shade, reduced sand temperature, reduced wind movement, and lowered evaporation rates from the sand surface and a catchment for wind blown seeds. This inturn further lowers rates of water loss from
leaves. The increase plant diversity it promotes enriches leaf litter and the accumulation of humus. This is the key to a better water holding capacity of dune soils, thus paving the way towards the expansion of bio diversity (Plant and Animal) and stabilisation of successive communities. This succession conforms to the following set pattern as long as intervening factors don't interrupt the process. What might some of these intervening factors be? The set pattern:
Pioneer Zone Primary stabilising plants eg grasses and herbaceous plants establishes the basis for
A Woodland or Scrub Zone with secondary stabilising plants consisting of Acacias, vines stunted trees, banksias and a few stunted herbs. These establish conditions conducive to the development of
Tertiary Stabilising Plant Zone of a) Heathland (low shrubs) where soil drainage is poor or relatively unprotected from sea wind); b) Forests (trees, high drainage and a history of protection from the sea)
Wetlands Mangroves, Melaleuca t tree swamps, Marsh plants such as sedges, succulent salt plants and salt couch occur where estuaries interrupt a dry succession.
Vegetation Degradation The foredune complex is a dynamic, resilient landform essential to the long-term stability of the dune system. However, dune vegetation may be degraded or damaged through both natural and artificial processes. Natural instability may be initiated by high energy storm events capable of breaching or scarping the foredune. Failure of the dune vegetation to recover from storm wave attack may result in dune erosion due to the effects of onshore winds mobilising bare sand in the form of blowouts. Perhaps the most important "natural" factor responsible for altering the structure of stable vegetation is fire, the effect of which is to reduce the presence of mature shrubs and trees, resulting in a grassland formation that in some locations is maintained by repeated firing. Once degraded, natural healing of dune vegetation may be inhibited by: ·Breakdown of the vegetation canopy which, when intact, deflects winds over it. ·Entry of salt-laden wind to the internal structure of the vegetation can expose sensitive vegetation elements (especially rainforest species) to salt-burn and resulting in irreversible degeneration of the vegetation binding the foredune. ·Continued pedestrian and vehicular traffic and other disturbance factors at a level sufficient to prevent recolonisation of the exposed surfaces by plants. Sand Drift Hazard Sand drift is caused by windborne sediment transport. All sandy beaches experience sand drift to a greater or lesser extent. Moreover, it is a seemingly slow moving and gradual process, but short episodes of strong wind can move surprisingly large volumes of sand. Sand drift is a serious problem along the NSW coastline. A recent study has shown that 5,600 ha, or 11% of the total area of coastal dunes, is completely bare of vegetation and is undergoing active drift. A further 4% of the total dune area is in a state of incipient destabilisation leading to unimpeded drifting. Sand drift creates a variety of hazards. At best drifting sand is a nuisance; at worst it represents a permanent loss of sand from the beach system and may completely overwhelm coastal developments. Detrimental effects include the abrasion of motor vehicles, buildings, vegetation and park and garden fittings; the burial of roadways, rail lines, agricultural land and coastal ecosystems; the blockage of street gutters and stormwater drains; and structural damage to buildings caused by forces imposed by the sand. The Importance Of Dune Vegetation In its natural state, dune vegetation provides an aerodynamic "cover" which deflects salt laden wind over the dunes. This minimises wind attack in dune areas and acts to trap any wind blown sand. Breakdown of the vegetation canopy can expose sensitive foredune species to salt burn (especially rainforest plants). This can lead to irreversible degradation of protective vegetation, which in turn leads to dune "blowouts". Wind velocities are faster through the throat of the blowout, which causes more sand to drift. Thus, the blowout grows in a seemingly unstoppable fashion and a moving "slipface" is formed at the rear of the dune. Figure 2: Dune Blowout and Slipface
The revegetation of exposed dune and beach areas is hindered by sand drift. Difficulties can also arise if exposed areas are first, remote from a source of nutrient supply (normally the hinddune and strandline areas), second, remote from the source of recolonising seedlings, and finally, exposed to a microclimate considerably harsher than that of hinddune areas. Factors Causing Sand Drift Sand drift in a coastal location is usually initiated by the degeneration or destruction of vegetation protecting the vital foredune. Common causes are foot and vehicle tracks devoid of vegetation running down the face of the dune. Studies have shown that as few as 50 vehicular passes per year are sufficient to prevent the revegetation of tracks. Sand blown inland is permanently lost to the beach system and is not available to assist in the natural rebuilding of the dune system following wave damage. This leaves the dune system in a weakened condition to combat the next series of storm waves, which in turn results in progressively more serious beach erosion and may ultimately lead to shoreline recession. Figure 3 Dwellings Threatened by Sand Drift, Anna Bay, New South Wales The graph below shows the change over the period 1967 to 1987 in the area of active sand drift along the New South Wales coast. The quite striking decrease in the drift area north of latitude 31o is largely attributable to invasion of these areas by bitou bush. Bitou is a vigorous coloniser. In addition to colonising bare areas, it also displaces native dune vegetation and destroys the habitat of native animals. In the long term, bitou provides a less resilient protection against wind and wave erosion than native vegetation. The removal of significant areas of bitou by storm attack or through biological or herbicidal control could trigger widespread sand drift in areas where it presently dominates dune vegetation. Figure 3: Changes in Area of Active Sand Drift Along the New South Wales Coastline, 1967 to 1987 2. Management Beaches, dunes and cliffs are natural buffer zones which protect the land from waves, winds, salt and extreme events. However, the impact of human use combined with the effects of coastal processes can damage the buffer zone and threaten natural habitats and human assets.
Dunes are subjected to many
pressures by people Blowout
Protection Of Our Dunes Whilst the beach like out tough outer layers of skin can be tolerant to intensive recreational use, careless pollution by leaching chemicals and oils can critically impair this first line of defence. Rubbish left on beaches especially of the non biodegradable variety can get back out to sea and there reach their damaging effects on wildlife eg plastic bags. Unlike the beach the frontal dune is extremely fragile. Destruction of its vegetation established by even moderate pedestrian use or grazing by domestic animals can quickly undo the evolutionary process of succession. Even tiny patches of frontal dune loss can leave much bigger areas open to destruction as strong on shore winds target the small discrepancy, like water finding the plug hole in the sink or bacteria pouring through a microscopic puncture wound in the skin. These winds can result in Blow out, then transgressive mobile dunes, resulting in a completely unstable dune system rapidly moving inland. Ecosystems adjacent inland establish by dune formation and stabilisation are extremely vulnerable to dune breakdown, sand swamping by the inland march of a transgressive dune kills established Woodland, Heathland and Forests. The whole process of succession has to be repeated all over again before their ecology can hope to re-establish it's original biodiversity unless these destroyed regions are nurtured, fertilised, re planted and stabilised artificially by us.
2.1 Revegetation Revegetation of dunes and cliffs restores a protective cover which stabilises dunes and eventually provides wildlife habitat. Local communities, conservation groups, Coast Action groups, 'Friends of' groups, Landcare groups are actively involved in revegetation works along the coast. Planting of local grasses, shrubs and trees, often propagated from local stock, has repaired many degraded areas and rehabilitated coastal habitats. Marram grass, originally from Europe, has been widely used around the world to successfully stabilise sand dunes. Whilst marram grass is easy to propagate and establish, it is an introduced species which is able to persist and spread at the expense of indigenous grasses. Alternatives to planting marram grass are now being used for dune rehabilitation. Local grasses and shrubs are planted in weed control mats manufactured from organic materials such as jute, or amongst brush matting such as prunings from indigenous species, although care should be taken to avoid prunings carrying unwanted seed. The matting helps to stabilise the soil while plants establish. Planting with indigenous species ensures that vegetation communities and habitats develop which support native wildlife. 2.2 Human Structures People have often seriously impacted on coastal dunes, failing to see that they are fragile and dynamic ecosystems subject to considerable natural change over time. Nowhere is this more evident than where unthinking, inappropriate and ill-planned human activities have resulted in coastal erosion. Coastal erosion becomes an issue when human investments and structures are threatened or lost. This is most likely to occur during severe storms such as those which affected the east coast in 1974. In this event up to 33 m of frontal dune retreat was recorded and damage to structures was reported from 110 beaches on the New South Wales north coast. Some activities of people promote coastal erosion. Beach access tracks are weak points in the frontal dune which tend to form blowouts. Stormwater drains discharging on the beach increase erosion and expose the dunes to wave attack. The hard surfaces of parking lots, roads, footpaths, sea walls, club houses, amenity blocks, ocean pools, dune fencing, launching ramps and piers all accelerate sand removal. These surfaces do not dissipate wave energy as well as the natural system and they are prone to damage in big storms. It is a natural human response to attempt to protect them. A common engineering response is to armour the beach with large stones or concrete tetrapods. This rarely works where the sand is deep because a positive feedback is involved when the immovable objects reflect more wave energy back to sea which erodes the sands below sea level, deepens the near shore profile and increases the energy available for erosion of the ‘protected’ coast. Engineering Solutions A number of engineering solutions are used for coastal protection. Construction of groynes and sea walls have been used in the past with varying success. Some have been successful, some require expensive ongoing maintenance and others have caused further problems. Artificial nourishment of beaches by transporting sand from elsewhere has restored many beaches. Groynes Groynes are fence-like structures, usually made of timber or rock which are built across a beach and into the water to trap sand. Where waves arrive at right angles to the shore, a series of groynes can trap sand and build small beaches between them. Where waves arrive at an angle to the shore and the beach is affected by longshore drifting, the first groyne will trap sand and starve the beach (and other groynes) further along the shore. Groynes can be designed to allow some sand to spill around the structure and minimise downstream erosion. Seawalls Seawalls may be timber, rock, steel or concrete and are constructed at the back of a beach to prevent a beach, dunes or cliff eroding and retreating landward. Whilst seawalls protect the landform directly behind it they can accelerate erosion at the end of the wall and cause erosion of the beach in front. Waves hit the wall then scour back sand from the beach. The beach becomes lower and flatter, the waves become larger, the scouring action becomes greater and the beach is eventually lost. By this time, the wall itself may be undermined by the increased impact of waves and require repairs. Beach Nourishment Artificial beach nourishment (renourishment) is the depositing of sand from elsewhere to replenish eroded beaches. Sand may be trucked from inland or dredged and pumped from offshore. The sand must be at least as coarse as the sand being replaced as fine sand will be carried away. Where beaches have been lost through longshore drifting it is likely an artificial beach will also be lost, so groynes may be used to trap drifting sand. Sediment eroded from cliffs replenishes many beaches therefore if sand is being transported from a beach by longshore drifting and erosion of the cliff is halted by revegetation works, the beach will be depleted. Interference with naturally occurring patterns of erosion and deposition can have undesired outcomes requiring further action. 2.3 Exotic Plants An exotic plant is a non-native plant. Selection of the wrong plant for any rehabilitation work can create weed problems or even change the natural form of the dunes. A weed is simply a plant which is growing outside of the ecosystem in which it evolved and has a tendency to take over. Even native plants can become weeds. About 50 plants are common coastal dune colonizers in Australia and some 25% of these are introduced. European Sea Rocket (Cakile maritima), was introduced to the West Australian coast in 1910. It reached the west coast of Tasmania and Victoria by 1935 and now occurs along the entire sandy coast of Tasmania, South Australia, Victoria and into New South Wales. It does not present a serious problem but it does largely replace native Dune Fescue (Festuca littoralis) and tends to create flatter dunes. In 1860 dune erosion was so severe in the Portland-Warrnambool area of Victoria that the Government Botanist advised the introduction of Marram Grass (Ammophilla arenaria) from Europe to stabilise the drifting sand. This species is still extensively used as a primary foredune coloniser despite the fact that it changes the shape of the dunes due to its upward growth habit which builds high hummocks of sand. In contrast, the native pioneer species Coastal Spinifex (Spinifex hirsutus), spreads across the foredune surface and creates a low mound or platform. Two natives shrubs from South Africa, Bitou Bush (Chrysanthemoides moniliferum ssp., moniliferum) and Boneseed (Chrysanthemoides moniliferum ssp.,rotundata) are extreme examples of exotic plants which were initially useful but have now reached weed status in coastal dune ecosystems. Introduced as a garden plant in 1858, Bitou Bush was first noted as a weed on Brighton Beach in Victoria in 1910. It also occurs in South Australia, West Australia and New South Wales. For twenty years from the 1950s its use as a dune stabiliser was encouraged by the Soil Conservation Service of New South Wales, but by 1970 it was acknowledged to be a serious weed. Today some 60% of the New South Wales Coast is invaded by it. Bitou Bush is spread by birds, rabbits and sand movement. It grows rapidly, crowds out native species, is a prolific seeder, and can tolerate salt, moderate wind shear and poor soil. As an exotic plant it has no local predators but this same characteristic allows us to attempt biological control. In 1992 the Bitou Tip-Moth was introduced by CSIRO and several other biological agents are being tested. Field experimentation has also shown that low doses of herbicides (glyphosphate) sprayed in winter can kill Bitou Bush without affecting dormant native species. To eradicate Bitou it is necessary to continue herbicide application and regular weeding programs over several years until seed stocks in the soil are exhausted and native species are established. Even native species can present weed problems under some conditions. For example; Coastal Tea-Tree (Leptospermum laevigatum), a native of southern New South Wales, was planted extensively on Stradbroke Island during the 1970’s as part of the post-mining rehabilitation and now forms dense monocultures which limit local species regrowth. Along the east coast the Golden Wreath Wattle (Acacia Saligna) was introduced from Western Australia. Plants out of place like this often exhibit the successful characteristics of weeds and may reduce local species biodiversity by taking over. To avoid these problems environmental scientists today attempt to ensure that the species mix used in rehabilitation is comparable to the original vegetation and they make pre-mining surveys to establish base line species composition and vegetation structure in order to set rehabilitation objectives. That is endemic species (those which have evolved in the local area, not just natives) are used. 2.4 Dunecare Dune Care began on the north coast of New South Wales under the auspices of the Soil Conservation Service in 1988. Initially 13 groups were established with the objectives of controlling erosion, rehabilitating dunes and beaches using local resources, educating the general public and land managers about the role and fragility of dune systems, and encouraging a land conservation ethic in the community. Some of the initial funding and publicity came from a Technical Jobs for Women Program, local media and local businesses. Since then Dune Care groups have flourished and have become part of the national program of Landcare. 2.5 Global Warming The wildcard in the pack of coastal dune management is the probability that sea levels are rising as a result of greenhouse induced climate warming. This is not just driven by melting ice caps but more subtly by any increase in sea surface temperature and the pattern will not be uniform either in time or space. Since 1992 sea level rise has been measured from the Topex satellite as 3mm per year on a global scale. At a local scale such rises are difficult to confirm but it is estimated that for every 1mm of sea level rise there can be 300mm of shoreline recession. This will inevitably impact on coastal structures especially in urban areas and we need to begin urgent planning to accommodate such change in the next few decades. 3. Decision-Making Planning for coastal development in New South Wales has been guided by the Coastal Council and more recently the Coastal Committee working under the NSW Coastal Protection Act (1979) and a Coastal Policy document. The Council reviewed this document in 1994 through an extensive public consultation process. Several coastal development inquiries have recently been held in other States and by the Commonwealth Government and it can no longer be said that we lack knowledge of the key coastal planning and management issues. The most important issues concerning beach and dune system management include: ·Effective zoning and local scale planning through instruments like Local Environment Plans. ·Stabilisation and restoration of existing erosion and weed invasion problems. ·Developing and applying ‘soft engineering’ techniques to specific dune management problems. ·Facilitating access to beaches through sensitive dune areas. ·Improving the communities' understanding of the role and significance of dune systems. ·Protecting representative natural dune environments in conservation reserves. ·Appropriate management and regulation of mining and recreational activities in coastal dune systems. Coastal dune systems are valuable natural resources. Whatever we do, we must recognise that sand dune systems are nature’s way of absorbing wave energy at the coast and that they are subject to constant dynamic change. All of our coastal dune management should be modelled on this premise.
The syllabus says you should learn about
· the geographical processes relevant to the issue
· the perceptions of different groups about the issue
· individual, group and government responses to the issue
· decision-making processes involved in the management of the issue
· management of the issue and implications for sustainability, social justice and equity
1. Geographic Processes
1.1 Oceanic Processes
Waves
Waves are created by wind blowing over the surface of water. Waves can travel thousands of kilometres until they reach shallow water where friction on the seabed causes them to slow down, become steeper, and eventually break upon the shore. Waves have a significant role in shaping the coastline, cutting into the softer rocks to carve out embayments, and leaving the more resistant rocks standing as headlands, cliffs and bluffs. Understanding waves and the currents they generate is important for beach safety, erosion control and the planning of developments and artificial structures both offshore and along the coast, such as seawalls, harbours, oil production platforms and buildings.
Rips and Longshore Currents
After a wave has broken on the shore, the water withdraws back to the sea either as an undertow (sheet flow over the sea floor) or in a rip current. Rip currents occur on many ocean beaches and can be extremely dangerous as a strong rip current can carry swimmers out to sea. The direction in which rip currents travel depend on the angle at which the waves approach the shore. The direction from which waves approach the shore affects where sand or other material is transported. When waves approach parallel to the shore, sand is moved in and out from a beach in cycles of fine and stormy weather, and there is generally no significant loss of sand from the area. Waves approaching at an angle to the shore generate currents which move sand in one direction along the shore. This process of longshore drifting can cause significant beach erosion, especially on long beaches uninterrupted by headlands, estuaries and inlets. Sand transported by longshore drifting is deposited further along the coast to form spits, which may later be washed away by a storm and deposited elsewhere. The western shore of Western Port and the north coast of Phillip Island are particularly subject to this as the south-westerly swell pushes into Western Port and moves sand along the shore.
Erosion, Weathering and Deposition
Erosion is the wearing away of cliffs, dunes, beaches and other landforms by the action of wind, rain or sea. Weathering is the breaking down of rock or soil by climatic conditions such as rainfall, hot sun or frost. Deposition is the accumulation of sediments, and is the process by which beaches, spits and mudflats are formed. Sand deposited by waves forms beaches, spits and barriers. Material eroded from cliffs also helps to build beaches. Mud and silt deposited by rivers builds mudflats on sheltered coasts. Waves are closely linked with weather and the processes of erosion and deposition along the coast. In stormy weather, waves plunge with great force upon sandy shores, pulling sand from the beach and withdrawing it to the sea. Waves attack cliffs, wearing away at the rock, breaking it down into sediments. After storms, backshore dunes may be cliffed and rocky outcrops appear on depleted beaches, yet after a period of calm weather the beach and dunes are often replenished with sand. In fine weather, spilling waves deliver sand to the shore creating and supplementing beaches, spits and sandy barriers. This cyclic pattern of cut (erosion) and fill (deposition) is constantly changing the shape of the coast in the short and longer term.
Extreme Events
Storm surges are a build up of water in the coastal zone by strong on-shore winds (winds blowing from the sea towards the land) and can cause severe erosion, especially when they coincide with high spring tides. A normal high tide is about 1.5-2m, a spring tide can be 3-4m and a storm surge 2-5m. If a storm surge combines with a spring tide. The sea level may be raised by 5-9m and have large 4-5m waves on top of that. This combination of conditions is rare, occurring perhaps about once every 20 years. It is also what causes the severe storm events which cause buildings to be washed into the sea.
1.2 Dune Processes
1.2.1 Formation
Primary dunes are derived from the beach and constructed where colonising vegetation becomes established. These are usually frontal dunes parallel to the beach. Secondary dunes develop as erosional features from the primary dunes. In contrast to the primary dunes, secondary dunes are often aligned parallel to the wind which formed them, and therefore the dunes lie at high angles to the beach.
The seaward side of most dunes have a relatively gentle slope of about 10 degrees and a lee slope or slip face which lies at about 32 degrees - the angle of repose for dry sand (ie. dry sand cannot be piled into a hill with a slope more than 32o). When winds are strong and persistent and vegetation is absent, secondary dune crests can move inland at 2 to 3m per year.
A well developed vegetation pattern is usually present inland from the beach across any dune sequence. Colonising plants such as sand spinifex grow on the beach berm and frontal dune in raw sand. Inland from this are the pioneer shrub species such as coastal wattle; then low woodlands of tea tree and coastal banksia; to denser, taller woodlands and forests of eucalypts.
Soil development also increases inland. In most dune systems the tall forest is seen as the end point in vegetation communities but other end points can be recognised where the systems are larger and older. At Cooloola in southern Queensland for example, the oldest inland dunes are covered in a low banksia scrub beyond the forest stage because soil nutrients have been leached from the older sands and this restricts plant growth. In places on Fraser Island and at Myall Lakes the most complex plant community are patches of rainforest where virtually all the soil nutrients are recycled from the litter layer.
The plant species present in any dune system vary from place to place but the pattern is consistent and can be explained in terms of the ecological concepts of zonation and succession. Zonation being the communities’ response to gradual changes in environmental conditions from the open coast inland, for example high wind shear, salt spray, and soil development. Dunes further from the beach are older and the concept of succession may apply if each part of the dune system has passed through all the development stages. Elements of succession are certainly evident on any prograding (advancing) dune system but we should acknowledge that the processes causing dune advancement today are not necessarily the same as those operating thousands of years ago.
1.2.2 Dune Dynamics
The Erosion and Accretion Cycle
The physics of wind erosion is fairly simple. It occurs wherever a sandy soil is exposed to wind shear - the sand drifts and moves as mobile dunes until it is impeded by vegetation or enters a stream or lake. Dune fields vary in size depending on wind conditions, the extent of protective vegetation, and the supply of new sand from the beach.
Sand supply at the beach depends on the tidal range, wave regime and the delivery of sand from rivers. Along most of the Australian coast river valleys were converted to deep estuaries by the Pleistocene sea level rise. Because the total sand supply is finite and these estuaries now trap river sand, coastal erosion is likely to occur where adjacent beaches are not being nourished by new sand. Each beach system behaves as a closed cell in which the sand on the beach and frontal dunes moves on and offshore over time depending on the wave regime. Longshore transport or exchange of sand around coastal headlands is relatively unimportant except in northern New South Wales and southern Queensland.
Waves are the most important process affecting the form of the beach and frontal dune. Two types of waves occur around the coast and their distribution varies seasonally and geographically (Figure 2). Swell waves are generated by storms thousands of kilometres away and follow great circle routes around the world. On the east coast they mainly come from the southeast, on the west and south coast they come from the southwest. Dominant swells are important because swell waves are constructive waves which steadily move sand onto the shore where it may be blown into dunes.
Storm waves may be generated by local storms and more distant tropical cyclones and can come from any offshore direction. They are steeper, higher, more closely spaced (shorter period) and more destructive than swell waves. In northern Australia tropical cyclones occur once or twice a year and although they have very high wind speeds waves seldom exceed 2.5m in height. Coasts facing the Southern Ocean suffer the impact of storm waves 5 to 6m high for up to 40% of the year.
A typical erosion/accretion cycle on the east coast sees winter erosion by storm waves followed by slow rebuilding of the beaches and frontal dunes by the summer swell. The body of sand affected by this cycle lies within the sweep zone. Figure 3 illustrates some of the main features of a typical coastal dune sequence and a sediment budget.
Soil Formation
Soil profiles formed in coastal sand dunes increase in depth and maturity with time. At the frontal dune there is no soil development. However in the hind dunes, litter begins to accumulate, a topsoil forms and any shell fragments in the sand are dissolved and leached away changing the soil pH from alkaline to acid. Moving inland through the taller and more diverse vegetation, podzol soil profiles develop in the inert quartz sands. Here the only other mineral components are the thin films of iron and aluminium oxides coating the sand grains and colouring them yellow. Plants growing on these low nutrient sites leach organic solutions into the soil as rainwater percolates from the surface. These solutions dissolve the oxides coating the sand grains and this iron/organic complex penetrates deeper into the soil where it breaks down and is precipitated as dark coloured ‘pans’ cementing the sand. The result of this podzolisation process is a distinctive soil profile. The topsoil is dominated by accumulating organic matter where most of the plant nutrients are stored and recycled. Beneath this lies a deep and intensely bleached white A2 horizon where all oxides have been stripped from the sand. Below this a complex of black or red-brown iron and organic pans is found. As time passes the soil profile deepens. The pans dissolve and move down through the profile forming bleached horizons many metres thick. The well known coloured sands of Rainbow Beach and Tewantin in southern Queensland are examples of giant podzol profiles.
1.2.3 Dune Vegetation
Figure 1: Coastal Sand Dune Vegetation
Vegetation And Coastal Processes
Vegetation is the key factor in dune stability and it is the vulnerability of dune vegetation that makes the dunes sensitive to impact. Examples of human activities that can damage dunes are: bush clearing; stock grazing; road building; construction; fuel storage; housing; most recreational activities; pedestrian and vehicular traffic; brush cutting and bushfires.
Role and Values
For the purposes of coastal dune management, it is convenient to divide dune vegetation into three groups. These groups are primary colonising species, (grasses and vines); secondary shrubs and transient species; and tertiary species (enduring trees and understorey).Dune vegetation is highly adapted to the salt laden winds of the coast, and maintains the foredunes by holding the sand already in the dune, trapping sand blown up from the beach, and aiding in the repair of damage inflicted on the dune either by natural phenomena or by human impact. The combination of dune height, dune shape and intact vegetation creates a protective system which directs salt-laden winds upwards and over the dune crest. As a result, salt sensitive vegetation communities, (including littoral rainforests), can establish in close proximity to the beach. Vegetation plays an important role in the stabilisation and formation of coastal dunes. Foreshore vegetation impacts on several of the sand transport pathways, and therefore influences the rate of shoreline recession and dune rebuilding. The strandline and beach vegetation is generally transient, being removed during storm events. Nevertheless these pioneer plants trap and hold windblown sand so that it does not damage plants on the relatively more stable foredune. The beach vegetation is mainly dominated by grasses. The foredune vegetation proper is usually composed of semi-permanent populations of herbs, shrubs and trees which stabilise the foredune sand mass. Sand trapped in the foredune acts as a reservoir of sand for the beach during periods of wave erosion, the foredune vegetation also buffers the effects of storm erosion.
Fore dunes act as Barriers against the action of waves, tides and salt wind and are a source of sand for the beach during periods of erosion. Their colonisation and consolidation by plants gives them a flexible function as sand traps and self consolidation after exposure to heavy storm waves. Fore dunes that are densely vegetated protect hind dune habitats and promote more complex and diverse plant communities.
Secondary and tertiary dunes are the products of uninterrupted succession. These provide further protection and the establishment of those critical ecological conditions conducive to promoting the most stable of all stages the coastal climax forest dominated by advanced flowering plants such as eucalypts, coastal apple and tuckeroos.
Whether they be of the stable uniform stretch of low dune characteristics of our northern NSW coastline or the gargantuan sand cliffs of Cooloola and the huge packed parabolic dunes of Stradbroke, Moreton and Fraser Islands transgressing inland as wind blown deltas of fine white sand called blowouts they all started in the same humble way. The little Hillock now receives bird droppings rich in nitrates and phosphates, good plant food. Inevitably some of these beach "pimples" will stop the wind blown march of a strange spider like tumble weed, the seed capsule of that hardiest of all grasses called spinifex.
Spinifex on Primary Dune
Soon after, it germinates to establish a root hold and spreads web like rhizomes (creeping stems) to capture the hillock and other rolling tumble weeds in it's first act of sand stabilisation and dune formation. These are the pioneer colonisers of sand dunes. Colonisers like Spinifex improve local conditions for other binders such as pig face and the goastfoot vine to join the battle for stabilisation. Together with time, isolation and minimal interference they help create the first thin skin of coastal protection called a primary dune.
Morning glory, another dune colonising plant
Coastal Wattle
Heathland plants
Mean while in front of the primary dune new mini dunes continue to form. Over time compounded by the dropping of sea levels they become new primary dunes relegating the original dune to a highly vegetated sand hill called a secondary dune. As sea levels continue to drop over the last glacial period, tertiary dunes form. The original dune has become a coastal forest heathland and it's extended swale a luxurious insect and bird rich heath land.
We must ask ourselves what would happen to the great spread of coastal urban development that have displaced so many sand dunes of their natural barrier systems if a reversal was to occur and sea levels started to rise? From the air a series of parallel beach ridges fringing the seaward edge of many coastal plains indicate a history of coastal advance and sea retreat. The highest sand hill in the world is Mt Tempest of Moreton Island it is 280 metres tall and is in fact a huge transgressive dune.
This whole evolutionary give and take process of dune formation and its protective establishment and sustenance of developing coastal forest is called succession, the journey of the dune. Like the adding and shedding of our skin over time the dunes and sea cliffs form a dynamic living security system that surround our continent. It nurtures the essential feedback processes of homeostasis that keeps coastal terrestrial eco systems in balance enabling them to operate autonomously with relative impunity. Our dune system also
The coastal forests, heathlands, wetlands and mangroves, sea grasses and salt marshes that this dune succession spawns promotes outgoing sediments of sand and detritus (waste organic matter) which unless contaminated by alien infusions of questionable origin and intent feed the capillaries of it's creek and river systems to fuel the oceans and ultimately refurbish the very basis for it's own continuously recycled and sustained existence.
Key members of nature's green dune squad and their significance
Higher up on the dunes it is salt winds that is the invasive force. Aerial parts of vegetation block the wind energy and cause sand to deposit around the vegetation. A characteristic of dune vegetation particularly grasses like spinnifex, pineapple sedge and dune carex is their ability to produce up right stems and sand trapping rhizomes that can grow firm roots in response to sand coverage. This sand deposition around plants results in increase height and width of the dune, a process known as plant induced dune expansion.
The ability of pioneer plants such as Spinifex and Pigface to hold wind blown sand on the frontal dunes helps create conditions which encourage the establishment of other communities such as Woodland Scrub (Acacias, Tuckeroos) Heathland (Banksias, Xerophytes such as Pea plants, Heath and Boronias and Melaleucas and Coastal Forest (Eucalypts and Angophora). All plants (Herbs, shrubs ,reeds, grasses, trees) are of equal importance in developing vegetation. Like the interplay between diverse cell types within the connective tissue meshwork of our skin underlay (dermis) that stops the outer skin from splitting and falling apart these plants enable dune stabilisation to continue indefinitely.
The whole process of dune formation and succession depends primarily on the pioneer plants and of all of these Spinifex is the chief. Spinifex hirsutus (The Captain Cook of the dunes). This is the most successful sand trapping dune coloniser and pioneer along our northern beaches. It provides the basis for a dune and establishes the first of a new set of environmental conditions conducive to the radiation of a more diverse vegetative cover.
These conditions include increased shade, reduced sand temperature, reduced wind movement, and lowered evaporation rates from the sand surface and a catchment for wind blown seeds. This inturn further lowers rates of water loss from
Pigface
Tuckeroo seeds
Insectivorous plant from Wallum heathland
Spinifex trailers showing sand-binding capacity
This succession conforms to the following set pattern as long as intervening factors don't interrupt the process.
What might some of these intervening factors be?
The set pattern:
Vegetation Degradation
The foredune complex is a dynamic, resilient landform essential to the long-term stability of the dune system. However, dune vegetation may be degraded or damaged through both natural and artificial processes. Natural instability may be initiated by high energy storm events capable of breaching or scarping the foredune. Failure of the dune vegetation to recover from storm wave attack may result in dune erosion due to the effects of onshore winds mobilising bare sand in the form of blowouts. Perhaps the most important "natural" factor responsible for altering the structure of stable vegetation is fire, the effect of which is to reduce the presence of mature shrubs and trees, resulting in a grassland formation that in some locations is maintained by repeated firing.
Once degraded, natural healing of dune vegetation may be inhibited by:
· Breakdown of the vegetation canopy which, when intact, deflects winds over it.
· Entry of salt-laden wind to the internal structure of the vegetation can expose sensitive vegetation elements (especially rainforest species) to salt-burn and resulting in irreversible degeneration of the vegetation binding the foredune.
· Continued pedestrian and vehicular traffic and other disturbance factors at a level sufficient to prevent recolonisation of the exposed surfaces by plants.
Sand Drift Hazard
Sand drift is caused by windborne sediment transport. All sandy beaches experience sand drift to a greater or lesser extent. Moreover, it is a seemingly slow moving and gradual process, but short episodes of strong wind can move surprisingly large volumes of sand. Sand drift is a serious problem along the NSW coastline. A recent study has shown that 5,600 ha, or 11% of the total area of coastal dunes, is completely bare of vegetation and is undergoing active drift. A further 4% of the total dune area is in a state of incipient destabilisation leading to unimpeded drifting. Sand drift creates a variety of hazards. At best drifting sand is a nuisance; at worst it represents a permanent loss of sand from the beach system and may completely overwhelm coastal developments. Detrimental effects include the abrasion of motor vehicles, buildings, vegetation and park and garden fittings; the burial of roadways, rail lines, agricultural land and coastal ecosystems; the blockage of street gutters and stormwater drains; and structural damage to buildings caused by forces imposed by the sand.
The Importance Of Dune Vegetation
In its natural state, dune vegetation provides an aerodynamic "cover" which deflects salt laden wind over the dunes. This minimises wind attack in dune areas and acts to trap any wind blown sand. Breakdown of the vegetation canopy can expose sensitive foredune species to salt burn (especially rainforest plants). This can lead to irreversible degradation of protective vegetation, which in turn leads to dune "blowouts". Wind velocities are faster through the throat of the blowout, which causes more sand to drift. Thus, the blowout grows in a seemingly unstoppable fashion and a moving "slipface" is formed at the rear of the dune.
Figure 2: Dune Blowout and Slipface
The revegetation of exposed dune and beach areas is hindered by sand drift. Difficulties can also arise if exposed areas are first, remote from a source of nutrient supply (normally the hinddune and strandline areas), second, remote from the source of recolonising seedlings, and finally, exposed to a microclimate considerably harsher than that of hinddune areas.
Factors Causing Sand Drift
Sand drift in a coastal location is usually initiated by the degeneration or destruction of vegetation protecting the vital foredune. Common causes are foot and vehicle tracks devoid of vegetation running down the face of the dune. Studies have shown that as few as 50 vehicular passes per year are sufficient to prevent the revegetation of tracks. Sand blown inland is permanently lost to the beach system and is not available to assist in the natural rebuilding of the dune system following wave damage. This leaves the dune system in a weakened condition to combat the next series of storm waves, which in turn results in progressively more serious beach erosion and may ultimately lead to shoreline recession.
Figure 3 Dwellings Threatened by Sand Drift, Anna Bay, New South Wales
The graph below shows the change over the period 1967 to 1987 in the area of active sand drift along the New South Wales coast. The quite striking decrease in the drift area north of latitude 31o is largely attributable to invasion of these areas by bitou bush. Bitou is a vigorous coloniser. In addition to colonising bare areas, it also displaces native dune vegetation and destroys the habitat of native animals. In the long term, bitou provides a less resilient protection against wind and wave erosion than native vegetation. The removal of significant areas of bitou by storm attack or through biological or herbicidal control could trigger widespread sand drift in areas where it presently dominates dune vegetation.
Figure 3: Changes in Area of Active Sand Drift Along the New South Wales Coastline, 1967 to 1987
2. Management
Beaches, dunes and cliffs are natural buffer zones which protect the land from waves, winds, salt and extreme events. However, the impact of human use combined with the effects of coastal processes can damage the buffer zone and threaten natural habitats and human assets.
Dunes are subjected to many
pressures by people
Blowout
Whilst the beach like out tough outer layers of skin can be tolerant to intensive recreational use, careless pollution by leaching chemicals and oils can critically impair this first line of defence. Rubbish left on beaches especially of the non biodegradable variety can get back out to sea and there reach their damaging effects on wildlife eg plastic bags. Unlike the beach the frontal dune is extremely fragile.
Destruction of its vegetation established by even moderate pedestrian use or grazing by domestic animals can quickly undo the evolutionary process of succession. Even tiny patches of frontal dune loss can leave much bigger areas open to destruction as strong on shore winds target the small discrepancy, like water finding the plug hole in the sink or bacteria pouring through a microscopic puncture wound in the skin.
These winds can result in Blow out, then transgressive mobile dunes, resulting in a completely unstable dune system rapidly moving inland. Ecosystems adjacent inland establish by dune formation and stabilisation are extremely vulnerable to dune breakdown, sand swamping by the inland march of a transgressive dune kills established Woodland, Heathland and Forests. The whole process of succession has to be repeated all over again before their ecology can hope to re-establish it's original biodiversity unless these destroyed regions are nurtured, fertilised, re planted and stabilised artificially by us.
2.1 Revegetation
Revegetation of dunes and cliffs restores a protective cover which stabilises dunes and eventually provides wildlife habitat. Local communities, conservation groups, Coast Action groups, 'Friends of' groups, Landcare groups are actively involved in revegetation works along the coast. Planting of local grasses, shrubs and trees, often propagated from local stock, has repaired many degraded areas and rehabilitated coastal habitats. Marram grass, originally from Europe, has been widely used around the world to successfully stabilise sand dunes. Whilst marram grass is easy to propagate and establish, it is an introduced species which is able to persist and spread at the expense of indigenous grasses. Alternatives to planting marram grass are now being used for dune rehabilitation. Local grasses and shrubs are planted in weed control mats manufactured from organic materials such as jute, or amongst brush matting such as prunings from indigenous species, although care should be taken to avoid prunings carrying unwanted seed. The matting helps to stabilise the soil while plants establish. Planting with indigenous species ensures that vegetation communities and habitats develop which support native wildlife.
2.2 Human Structures
People have often seriously impacted on coastal dunes, failing to see that they are fragile and dynamic ecosystems subject to considerable natural change over time. Nowhere is this more evident than where unthinking, inappropriate and ill-planned human activities have resulted in coastal erosion. Coastal erosion becomes an issue when human investments and structures are threatened or lost. This is most likely to occur during severe storms such as those which affected the east coast in 1974. In this event up to 33 m of frontal dune retreat was recorded and damage to structures was reported from 110 beaches on the New South Wales north coast. Some activities of people promote coastal erosion. Beach access tracks are weak points in the frontal dune which tend to form blowouts. Stormwater drains discharging on the beach increase erosion and expose the dunes to wave attack. The hard surfaces of parking lots, roads, footpaths, sea walls, club houses, amenity blocks, ocean pools, dune fencing, launching ramps and piers all accelerate sand removal.
These surfaces do not dissipate wave energy as well as the natural system and they are prone to damage in big storms. It is a natural human response to attempt to protect them. A common engineering response is to armour the beach with large stones or concrete tetrapods. This rarely works where the sand is deep because a positive feedback is involved when the immovable objects reflect more wave energy back to sea which erodes the sands below sea level, deepens the near shore profile and increases the energy available for erosion of the ‘protected’ coast.
Engineering Solutions
A number of engineering solutions are used for coastal protection. Construction of groynes and sea walls have been used in the past with varying success. Some have been successful, some require expensive ongoing maintenance and others have caused further problems. Artificial nourishment of beaches by transporting sand from elsewhere has restored many beaches.
Groynes
Groynes are fence-like structures, usually made of timber or rock which are built across a beach and into the water to trap sand. Where waves arrive at right angles to the shore, a series of groynes can trap sand and build small beaches between them. Where waves arrive at an angle to the shore and the beach is affected by longshore drifting, the first groyne will trap sand and starve the beach (and other groynes) further along the shore. Groynes can be designed to allow some sand to spill around the structure and minimise downstream erosion.
Seawalls
Seawalls may be timber, rock, steel or concrete and are constructed at the back of a beach to prevent a beach, dunes or cliff eroding and retreating landward. Whilst seawalls protect the landform directly behind it they can accelerate erosion at the end of the wall and cause erosion of the beach in front. Waves hit the wall then scour back sand from the beach. The beach becomes lower and flatter, the waves become larger, the scouring action becomes greater and the beach is eventually lost. By this time, the wall itself may be undermined by the increased impact of waves and require repairs.
Beach Nourishment
Artificial beach nourishment (renourishment) is the depositing of sand from elsewhere to replenish eroded beaches. Sand may be trucked from inland or dredged and pumped from offshore. The sand must be at least as coarse as the sand being replaced as fine sand will be carried away. Where beaches have been lost through longshore drifting it is likely an artificial beach will also be lost, so groynes may be used to trap drifting sand. Sediment eroded from cliffs replenishes many beaches therefore if sand is being transported from a beach by longshore drifting and erosion of the cliff is halted by revegetation works, the beach will be depleted. Interference with naturally occurring patterns of erosion and deposition can have undesired outcomes requiring further action.
2.3 Exotic Plants
An exotic plant is a non-native plant. Selection of the wrong plant for any rehabilitation work can create weed problems or even change the natural form of the dunes. A weed is simply a plant which is growing outside of the ecosystem in which it evolved and has a tendency to take over. Even native plants can become weeds. About 50 plants are common coastal dune colonizers in Australia and some 25% of these are introduced. European Sea Rocket (Cakile maritima), was introduced to the West Australian coast in 1910. It reached the west coast of Tasmania and Victoria by 1935 and now occurs along the entire sandy coast of Tasmania, South Australia, Victoria and into New South Wales. It does not present a serious problem but it does largely replace native Dune Fescue (Festuca littoralis) and tends to create flatter dunes.
In 1860 dune erosion was so severe in the Portland-Warrnambool area of Victoria that the Government Botanist advised the introduction of Marram Grass (Ammophilla arenaria) from Europe to stabilise the drifting sand. This species is still extensively used as a primary foredune coloniser despite the fact that it changes the shape of the dunes due to its upward growth habit which builds high hummocks of sand. In contrast, the native pioneer species Coastal Spinifex (Spinifex hirsutus), spreads across the foredune surface and creates a low mound or platform.
Two natives shrubs from South Africa, Bitou Bush (Chrysanthemoides moniliferum ssp., moniliferum) and Boneseed (Chrysanthemoides moniliferum ssp.,rotundata) are extreme examples of exotic plants which were initially useful but have now reached weed status in coastal dune ecosystems. Introduced as a garden plant in 1858, Bitou Bush was first noted as a weed on Brighton Beach in Victoria in 1910. It also occurs in South Australia, West Australia and New South Wales. For twenty years from the 1950s its use as a dune stabiliser was encouraged by the Soil Conservation Service of New South Wales, but by 1970 it was acknowledged to be a serious weed.
Today some 60% of the New South Wales Coast is invaded by it. Bitou Bush is spread by birds, rabbits and sand movement. It grows rapidly, crowds out native species, is a prolific seeder, and can tolerate salt, moderate wind shear and poor soil. As an exotic plant it has no local predators but this same characteristic allows us to attempt biological control. In 1992 the Bitou Tip-Moth was introduced by CSIRO and several other biological agents are being tested. Field experimentation has also shown that low doses of herbicides (glyphosphate) sprayed in winter can kill Bitou Bush without affecting dormant native species. To eradicate Bitou it is necessary to continue herbicide application and regular weeding programs over several years until seed stocks in the soil are exhausted and native species are established.
Even native species can present weed problems under some conditions. For example; Coastal Tea-Tree (Leptospermum laevigatum), a native of southern New South Wales, was planted extensively on Stradbroke Island during the 1970’s as part of the post-mining rehabilitation and now forms dense monocultures which limit local species regrowth. Along the east coast the Golden Wreath Wattle (Acacia Saligna) was introduced from Western Australia. Plants out of place like this often exhibit the successful characteristics of weeds and may reduce local species biodiversity by taking over.
To avoid these problems environmental scientists today attempt to ensure that the species mix used in rehabilitation is comparable to the original vegetation and they make pre-mining surveys to establish base line species composition and vegetation structure in order to set rehabilitation objectives. That is endemic species (those which have evolved in the local area, not just natives) are used.
2.4 Dunecare
Dune Care began on the north coast of New South Wales under the auspices of the Soil Conservation Service in 1988. Initially 13 groups were established with the objectives of controlling erosion, rehabilitating dunes and beaches using local resources, educating the general public and land managers about the role and fragility of dune systems, and encouraging a land conservation ethic in the community. Some of the initial funding and publicity came from a Technical Jobs for Women Program, local media and local businesses. Since then Dune Care groups have flourished and have become part of the national program of Landcare.
2.5 Global Warming
The wildcard in the pack of coastal dune management is the probability that sea levels are rising as a result of greenhouse induced climate warming. This is not just driven by melting ice caps but more subtly by any increase in sea surface temperature and the pattern will not be uniform either in time or space. Since 1992 sea level rise has been measured from the Topex satellite as 3mm per year on a global scale.
At a local scale such rises are difficult to confirm but it is estimated that for every 1mm of sea level rise there can be 300mm of shoreline recession. This will inevitably impact on coastal structures especially in urban areas and we need to begin urgent planning to accommodate such change in the next few decades.
3. Decision-Making
Planning for coastal development in New South Wales has been guided by the Coastal Council and more recently the Coastal Committee working under the NSW Coastal Protection Act (1979) and a Coastal Policy document. The Council reviewed this document in 1994 through an extensive public consultation process. Several coastal development inquiries have recently been held in other States and by the Commonwealth Government and it can no longer be said that we lack knowledge of the key coastal planning and management issues.
The most important issues concerning beach and dune system management include:
· Effective zoning and local scale planning through instruments like Local Environment Plans.
· Stabilisation and restoration of existing erosion and weed invasion problems.
· Developing and applying ‘soft engineering’ techniques to specific dune management problems.
· Facilitating access to beaches through sensitive dune areas.
· Improving the communities' understanding of the role and significance of dune systems.
· Protecting representative natural dune environments in conservation reserves.
· Appropriate management and regulation of mining and recreational activities in coastal dune systems.
Coastal dune systems are valuable natural resources. Whatever we do, we must recognise that sand dune systems are nature’s way of absorbing wave energy at the coast and that they are subject to constant dynamic change. All of our coastal dune management should be modelled on this premise.